1. Field of the Invention
The field of the invention is step motor circuits and more particularly those circuits which include means for preventing voltage spikes which tend to occur at the ends of the step motor windings.
2. Description of the Prior Art
In the normal operation of a step motor circuit voltage spikes periodically occur at the end of the step motor windings. These voltage spikes are caused by the leakage inductance through a winding when an associated contact switch is instantaneously opened. These leakage inductance spikes tend to damage the operating switches (usually transistors). To minimize damage, more expensive, high-voltage-tolerant components are required. Further, these spikes interfere generally with the overall operation of the motor, and their dissipation generates substantial heat loss which results in wasted power consumption.
Leenhouts U.S. Pat. No. 4,121,144 discloses a step motor cicruit which utilizes an RC circuit to partially solve the leakage inductance problem. The RC circuit is used as a source of current to prevent the voltage spikes from reaching undesirable proportions. In Leenhouts the current path used to "drain" the voltage spikes flows through the RC circuit, the winding, a diode and a second switch which is closed. Although Leenhouts does partially solve the despiking problem, additional problems are created in that (1) the RC circuit itself interferes with the overall operation of the switching transistors; (2) heat loss in the resistor of the RC circuit is power inefficient, and (3) because the RC circuit does not react instantaneously, the voltage spikes are not completely maintained within the voltage limits of the circuit components.